You may want to reconsider some of your ratings for Monster Slayer, since you made a major mistake on Slayer's Prey. It lasts until you take a rest, and can be used unlimited times but only affects 1 enemy at a time.
I read it over again, and you're right. That bumps Slayer's Prey to black. I still rate the subclass purple as a whole, though, due to a weak bonus spell list and the highly situational Lv. 11 feature that comes at the same level that all other Rangers are getting major boosts to their offense.